Automatic telephone system.



T. G. MARTIN. AUTOMATIC TELEPHONE SYSTEM; APPLICATION FILED FEB. 26,i914.

1,289,853; Patented Dec. 31, 191g a suns-sneer 1.

O Y. Q w

T'. G. MARTiN. AUTOMATIC TELEPHONE SYSTEM. l

APPLmATxQN FILED FEB. 2e. 1914. f

Patented Dec. 81,1918.

3 SHEETS-SHEET 2.

T. G. MARTIN. AUTOMATIC TELEPHONE SYSTEM.

APPLICATION FILED FEBZG. I9I4. I l,28f9,853. Patented De0.31,1918.

MTA/56555 generators l rand ll". vcontacts' is connected to a ringingdevice H lshown in Fig'. l.

vice H. The iirst set of contacts, namely 5l, 5:2 and is connected tothe ringing device El', which device alternately applies .inging currentto` the line from the generators lt and R2. 'The second set of contactsis connected to a ringing device H (not shown), which device alternatelyapplies 'ringing current to the line from the cnerators lt and lli". Thethird set oli con tants is connected to a ringing device lil (notshown), which device alternately applies ringing; current to the linefrom the The fourth set of {not shown), which device alternately appliesringing;l current to the fline from 'the generators lli and l. The litthset of contacts is connected to a ringing-'device H (not shown), whichdevice alternately ap# plies ringing` current to the line from thegenerators l and llt. tacts isvconnected to a ringing device lll {notshown), which deviceralternately applies ringing current to the linefrom the generators lis and lift. `From the foregoing; it will beapparent that have connected the diercnti generators to the dili'crcntdevices ll" in such a. manner as to allow any calling subscriber tosigna-l any other subscriber on. his own line. For example, ii thesubscriber at substation ist wishes toI cali the subscriber atsubstation A." he will cause his line to he connected to a selectivedevice H and then operate itfto rotate onto the third set ot contacts39, l0 and el, which contacts are connected with a device H whichlsupplies current Iroin tnegenerators R and R4. The ringing current fromthe generator R will operate the called' siibstation bells, whileL Theconnector J is 'of the same general type of connector switch as thatdisclosed in United States Let-ters Patent N o. 815,176, grant-ed March13, 1906, to Keith, Erickson and Erickson, thecircuits, however, being,-modified to adapt the switch to operate in e system in which the centraloliice apparatus is controlled by impulses delivered o" the two sides ofthe line in series.' The circnits are also modified t.) adapt the switchin embodying prim "n Lilie lh`- selec' The sixth set of con-l iscont-rolled by the-private magnet 70 in the usual and well-known manner.The magnet 7l is the usual release magnet. rllhe line relay 72 isconnected to the line through the reversing` springs of the socallcdbackbridge relay 73. rThe. ringing relay 7l is the ineansvby -whichringing current-is ap- 'plied to the called line or trunk. The busysignaling' machine L, as represented herein,

comprises an induction coil or transformer,

the priinarywinding ot' which is included in a local circuit with abattery and an interrupter ot' high frequency. Through the medium ofthis interrnpter an interrupted currentis supplied to the primarywindin-o, which induces an alternating current in the secondary Winding,whereby, when said winding is connected with the snbscribers line, abuzzingsound is heard in the receiver. i Y The substation A and the lineswitchI C are similar in all respects to the substation A and the lineswitch C.

Associated with the trunk line leading from the first selector banks tothe repeaters l (which are located in another exchange) is a group ofholding relays (only one of said relays being shown) which are for thepurpose of supplying` a holding` ground to" the first selector and alsofor the purpose or" puttingv a. guarding ground potential on vtheprivate bank contact'of the trunk lines in the banks or' all the rstselectors that have access to the trunk lines troni the first exchange.VThe repeaters l place a guardcontacts ol the trunk lines in the banks ofthe selectors which liaveaccess to said trunk lines 'from the exchangein which the said repeaters are located.

A general description ofthe apparatus having been given, it will now beexplained how one of the subscribers on the party-line may signalanother subscriber on the saine line` For example, should the subscriberat substation A call'the subscriber at the substation A2, both 0i" whomare on the saine line. lt is assumed herein that the second level ol theirstLselectors is reserved for revertingcally purposes `For this purposethere is connected tothe contacts or' this level plurality of trunklines, each of which connect to a ring-bach selective device ll.

YUpon the removal of the receiverflroni them itch-hooi: at thesubstation A preparaa call, an ener line i i loci' yi (l inserta tionover the heavy conductorsthrough the wipers 25 and 27 to the ring-backSelective device H. is soon as the connection is cornpleted with thesclectivedevicc H, the line relay 15 becomes energized. rThis circuitextends from ground G5 through the lower winding ot' tl relay 15,springs 101 and 102 of the switching relay 117, conductor 108, wiper 27,thence over tue heavy conductors to and through the substation A', bachover the heavy conductors, wiper 25, conductor 101, springs 105 and 100and the upper .winch ing ot the relay 45 to the battery lead 100. Therelay l5, upon energizing, closes a cir-- cuit through the Slow relayThis circuit extends 'troni -round G through the springs 10T and 10S andthe relay 40 to the battery lead 100. rl`he relay 4G, upon energizing,opens the circuit ort the release magnet i3 at the springs 10S' and 110and closes a holding `ground Vlos the relay 80 ol the selector E and thewinding' 12 ot the line switch Said holding' circuit extends :trom,ground Gl' througijh the springs 111, wiper 20 and the springs Sile andoi? the relay thence through the relay 80 to the battery lead 100. Theholding" ground tor the winding` 12 eX- tends from ,ground (il over thecircuit just traced to the point 112, thence through the normal springs93 and interrupler ,springs 91 and 00, springs 21 and 1T and thewinding),4 12 to the battery lead 100. The apparatus is now in readinessfor the Yoperation o'lI the dial i'or the second d T,it lhe ring-haeltselective device lil must now operate to select one ot the ringingdevices It will be reen trein the foregoing' descrip tion that thecoi'itacts 51` and (the iirst set o't contacts or the second baul;level) conneet with the ringing device il", which device is connectedwith the generators R and R2; and as the subscriber il wishes to callthe subscriber A2, he will next call the digit l. which will connect hisline with the first ringing device il.

rl'he operation oli' the calling device for the second digit 1 opens thecircuit ot the relay 45 once. rl'he slow actinga relay does notdeiinergize during the momentary interruptions ot its circuit by therelay 15, thereby preventing' tlie lirst selector l0 and the linelswitch C troni releasing. One impulse olf current is transmitted overthe circuit extending from ground G through the contact of springs 10?vand 113, s prihgsldf and 114, relay rotary magnet 12 und the side switchwiper 115 (in lirst position.) to the battery lead 100. Therotaryrnagnet receiving one impulse of current over said circuitloperates to rotate the switch shaft and wipers one step onto thecontacts 51, and 53. The, oil-normal springs le and 116 close as soon asthe shaft is rotated one step, but the circuit of the release mag net 18still remains open at the springs 110 )the battery lead 100. i

and 109. The slow acting relay 18 is maintained in its energizedposition while iinpulses are. being transmitted through it to the rotarymagnet, and in its energized position closes a circuit extending fromground GS through the contact of springs 118 and the relay 119 to thebattery lead 100. As soon as the relay 18 denergizes, the circuit of therelay 11S) is broken at the Contact of springs 118. rflic relay 119denergizing allows the side switch to Step to second position inwell-hnown manner.

The subscriber ii now replaces his re ceiver upon the switch-hook.Theline relay l5 then deiinergizes, its circuit being open at thecontact ot springs 120 and 121 of the substation A. The relay l-( beingslow acting does not immediately rfleinergize, thus maintaining therela)Y 80 of the. selector E and the .vinding;` 12 ot the line switch Cenergized until a new hol-ding circuit is completed. The relay upondeiincrgizing, closes a circuit e\ttmtling` from ground tl through thecont et ot springs 107 and 11i-3, sl'irings 10S) and ill, relaysr 1S and122 in. series and the side switch wiper 115 (in second position) to thebattery lead 100. The relay 18, upon energizing, again closes a circuitthrough the relay 110 to the battery lead 100, the relay 11S) holdinglthe side switch in second position. upon energizing, closes a circuitextending 'from ground lit" through the contact ot' springs 12J-l andthe slow actingy relay 46 to '"hc relay #113 thus remains ei'iergized,inaintainingy the relay 8G of the selector l0 and the winding 12 of theline switch t" energized. At the same time G10 (Fig. 2) through1 therelay 125, bank contact 53, wiper 3T, contact o1u springs 120 and therelay -lT to the battery lead 100. rlh'e rela)Y is et' a high resistanceand therefore the relay 4T does not receive suitticient current tooperatively energize. The relay t7 docs, ho\\*e\'er, receivesutlicientcun rent when in series with the relay #t9 or the relay 5l) tooperativelyenergize. The relay 12.3, upon energizing, closes a circuiteX- tentling' from ground il at. the solenoid l through the wiper 12T,contact 125%, righthand winding' oii the relay 129 and the springs 130and 1211 to the battery lead 100. The relay 120 becmninc` operativelyener- 5 "ed completes a circuit extending" 'from `ground G though thelower and upper windings ot the solenoid 132, contact ot springs 131 and133 and contactot the springs 130 anc 131 to the battery lead 100. Alocking;r Circuit for the relay 129 extends from gro/und G12 through thesprings 135 and 130, "right-hand Winding of the relay 129 and contact ofsprings 130 and 131 to the battery lead 100. energizing, raises itsshaft 137 and Wiper rEhe solenoid 132,11pouI The relay 122,`

CII

the generator 1:?? to point wlmi@ lie wipe? en?, niicuii; extending fromgronnd @u i a@nga t 138, wiperflQ', contact i397 leinnding of the relayi291, spin the sp"'ngs i3() and 13E. v lead 10C. The rela-5f ll bei?izillyh'ound relay, ills, Mfm'vi tubi@ Ve'flmnl Winding namlali? sindlli@ right-hand winding, .fm1 ne ieis. li' denQI-gizes. Thedeiiaim'izninii :mimn lik ay 129 breaksiis-own locking fore lie @omelziK' unit and also the ii'cnit'f the snlenid ;f, is iestnifecl, is 1the dis* Wlieicugvon 'the slnift stalls in move. slowly cli '29 m" l uin@ Calling line, (lm-"1" unifoim wie of l Sn :e i1 nioinonta'y Qn- 7ingdevice 'l 9 which may lin im f inin'm type. "Vl f winni' ci normal i ncircuit liion' gli lm I i taci;

The inlays +29 auf if? operatively @I i ai series, the Arelay 4:7 L0isconnc in@ re from the conducsf and un connect Said soncluctcrs to tlmwinni@ and 52, respectively. relay mi, npnn energizing, up ringingeurent2 to the cnnfluctni's j The. mili of lili@ ringing @airvent tends fromone Lel-Bninal 0"? the l?? tlii'owf the spiings affcominci; M5, bankConac smniac Sinin M8 and 02? cnn. and hes-nj nnl ccnducnis t@ o' oftin-t substation lin@ 150, the diilient balls 'land @onclensers fr@ andl wei,

substation line 'i owe' nea 'y i'nnl: conduct is, nonmict@ spii and 105,canin and' die conductor lfl back to im@ side of lie gena-Mor R2. 'Thebells il substaiinn A? are the only ones lia; 'espomL tn ringing current,fromtlie generator R2. Therefoie, the subscriber A is-signalefl t at avconnection is desired, llw slurf@ l2? li* l and called swlnntinns A lcalled subscriber eipnml by "D- his rnc v inc-simi honk. dus @comi 0i"as Sonn thai 7 ringing relay neen@ I? he pr vk gne? l d :a nifl the sid@switcn i0 sie@ in tliii'fl pnsltion. l The calling'subscribers bellceasing lo ring is an inclictin to him lmt the Called subscribe? has zumnd. Nn then ieznm'es his MN d i ing carried the Wiper 'past the Contactsl :im-i, me zxlknig um 2r H3, le relays 1'2" and l-Sl L l rough ne"windings o;

Wipe? if?" nml', engages lie Contact@ 115 154., vin a circuit amendingfrom l glie zeeni' @flic am 12 vlnfonh zne' springs 13 and l soui byEmmi A if mais wiper l2?, conac' and i e ieboth the calling and lle e@im iii 0I" ille slow :min :i relay'. 1V

' exeitli the lm @11V lead lOFL Bofill inlay 50 opn'atively enei'gzefl,lie with 'die saine iesul as before st;

el and inlay to apply ringing cuii'en' the t freni the gianni-nini' R lothe Conductm-q f 145 if) tlm subs'ation lino, over tltraced Circuit. ilrespond to said signaling; om Signa* f' allie Subscriber at lll/i edsulgwf reassess. c

upon the contact. i is opened and the ma@ neti permit-ted to denergize.During'the energization of the release magnet 43, con# tact between thesprings 156 was closed, thus 5 supplying` a guarding ground potential tothe bank contact 157 during the release of the selective device H. Thedenergization of the relay 86 01"' the selector E completes acircuitextending from ground Gr3 through the springs 82 and springs 8i and 85,springs SO and-8l7 oil-normal springs 32 and 315 and the release magnet31 to the battery lead lOO. The magnet 31, upon energizing', permits theshafto the selector to be restored to normal position, whereupon thecontact lmtwcen the springs and 33 is opened and the release magnetpermitted to 'lcincrgize. The energization et the release magnet 3lcloses the contact between the springs 155%. thus completing a guardingpo` tcntial lo the bank contact Qlduring' the release ci" the switch H.rllhe relajvY 46, upon decncrgrizing, also opens the circuit of theholding winding 12 of the line switch C and the line switch is permittedto release.

elcrring to Fig'. 2.-the conductor 159 is shown dotted. it the contactpoint 160 is joined to the con-- tact point l6l. by means of theconductor the relay -ll will be energized twice and the relav Ti() onceduring the return motion ot the shaft 137 to normal position. Therefore.the called subscriber will be signa-lcd twice. while the callingsubscriber will be signaled only once. l it is now apparent that adifferent method of zgroupingr of the grencraters will be necessary. Thegenerators will be connected to the different ringing' devices l-l insuch a manner as to aiiord any calling' subscriber access to aringinsr'device ll. which device will signal the called snbscribcz"sbells twice while it issignalinn the alling subscrihei"s bells onlyonce. l". will therefore connect the generators to the twelve differentringing devices in the following;` manner: The' first ringing device His connected to the generators it and R2,`the second device H to thegenerators E and E3. the third devicelH 'to the generators it and R4,the toiiirth device Hf to the genera,- tors T2 and R', the fifth deviceto generators R2 and its, the sixth device il to the generators Aandf7?. the seventh device il to the generators l?? and it", the 5 eighthdevice H to the generators and il?, the ninth device H to the generatorsand lili, the tenth device H to the generators ll and it', the eleventhdevice H to the generators Rf1 and R2 and the twelfth device H tothegenerators R4 and R3. is there are now twelve different ringing de!vices il', we must. connect two levels of the first selectin' banks tothe selective devices il, ontlei'el oi said selective ldevices il havinga-L-ccss to a partof the ringing relay delt will be readily seen'that-.f

oi the selector E energized.

' vices H and the other level having,` access to the remaining ringingrelay devicesrllit'.

It having been explained how `a `rwrting; call may be made, I will nowexplain how thevsubscriber A may call 'the subscriber A, Whose number is4201. The substation is one ot a number of similar substations whosenormals terminate in. bank contacts in the tenth level of the connectorswitch .l1 rEhe said tenth level of said connector may he used where abusiness house or private branch exchange, for instance, desires to havea plurali@7 of trunk lines extending to the main otlice,but wishes to begiven only one number. Y Y

Since the operation oi? the line switch C and the selector E havehereinbefore been described'. we' will assume @that the sabscriber Afhas removed his receiver and rotated his dial for the irst digit 4.connectinn4 the wipers 26 and 9,7 of the first selector ill with thebank contams 62. 163 and 16%-, As soon as the connection is completedwith the conductors 165, 166 and 1,67. .the Yline r lav 168 of theselector F is energized. rilhis circuit extends from ground G15 throughthe lower winding ofthe relai,r 168, springs 169 and 170 ofthe relaylflrcon ductor 167, bank contact 164-, wiper 27. over lthe hear;vconductor to the substation rlhe relay 174, upon energizing, closes a lcircuit extend-ing from ground G through the .springs 1.77 and 128.conductor 166. bank contact 163 and the wiper Q6. to the point H2. atwhich point the circuit divides, one

path extending' through the Contact oli springs 9i and 98 and the relay86 to the battery lead 100, maintaining the relai,7 The other branchextencs 'from the point through l the'springs 93 and $2, springs 91 and9.

hank contact 2l. wiper l? and the winding l2 to the lead iOO,holdinggthc .vind ine; of the line switch @energized relav 174i alsoprovides a guarding' ground. potenti al extending from ground G.through.

v the springs il?? and 1'78, conductor 166, hanli contact 163 and thenceto the corresponding-2,r

contact in the banket each selector switch. which has access to theselector F.

The apparatus is now in position to' re ceive impulses for the seconddigit 2. The action-oi thefselector 1l" heini: similar to that of theselector E. a, description of the same will not be given in detail. Thesubscriber 'at the substation A new operates his dial and 173 and the i.j :ming a. rotary wel, the shaft 0p- .Gnfncor 26:5, 0. b lf. com prisms'.192

cmmeciior @n .Ween .the sprin Het n emeg; fo close con Contact betw 2lias soon as he shaft 90 and sin'.

l. bank C0 nd 2G() thus m 13o f i Lacasse completiidy which extends fromground G' through the springs 209 and 220, interrupter springs 221,springs 222, spring 215 and 216,

' springs 218 and 217, rotary magnet 65 and the side switch wiper 66 (insecond position) t0 battery B. The rotary magnet, upon energizing,operates to step the shaft wipers one step onto the next bank contact.It also together. Should all the trunk lines be busy,

the calling subscriber will lie-given the busy signal in the followingmanner: As soon as the wiper 63 engages a grounded contact, a circuit iscompleted which extends from the guarding ground potential at the bankcontact through the wiper 63` springs 224 and 225, springs 227 and 226,relay 228 and the side switch wiper 66 (in second position) to batteryB. The relay 228, upon energizing, completes a locking circuit Jforitself which extends from ground G21 through the olfnormal springs 229and 230, springs 231 l and 225, springs 227 and 226, relay 228 and theside switch wiper 66 (in second position) to batteryB. At the moment therelay n228 y is energized the circuit of the rotary magnet is openedatthe springs 215 and 216, thus preventing the rotary magnet from beingoperated. The Vprivate magnet. remains energized and locks thefsideswitch in second position. The locking circuit for said private magnetextends from ground G21L through the off-normal springs 229 and 230,springs 231 and 232. left-hand winding of the private magnet and therelay 208 to battery B. The calling subscriber is given Athe busy signalover the following circuit .from tl'e busy signalinginachine L throughthe. springs 23-3, side switch wiper 69 (in `second position), condenser234, over .the heavy conductor through the substation A and over thereturn heavy conductor back to l the busy machine L. Assuming that thetrunk 'lines were not allL engagedand that the private wiper 60 foundalt idle Contact, the private magnet 70 denergizes and allows, the sideswitch to pass from second to third position. By the engagement of theside switch wipers 68 and 69 with their third position contact points,and by the denergization of the relay 208, the line connection islcompleted u ith the called subscriber,.as shown by the heavyconductors.l The relay 208' controls contacts in the line circuit and.made slow acting so asito pre-vent the completion oit the tine conn tionuntil the cubetti winding line ly has had time to operate its armature236 to disconnect the line relay 237 from the line. closed by theengagement ot. the side switch wiper 67 ofthe connector J with itsthirdposition contact point, said circuit extend-r ing from ground G23through the side switch wiper 67, private wiper 60, bank contact 4219,conductor 238 and the winding 235 to 'battery B. The connection ofground G23 with lthe connector bank `contact 219 ofI the called llineprevents the called line from being seized by any other connectorswitch.-

The circuit of the winding 235 is By the engagement of the side switchwiper 66 with its third-position contact point, the circuit of .theringer relay 74 is closed. This circuit extends from ground G24 throughthe interrupter 240, relay 74, springs 241 and the side switch wiper 66(in third position) to battery B. The ringingrelay, upon en.

ergizing, disconnects the calling linefrom.

the called line and bridges the ringing curzur rent generator R acrossthe called line tov f signal the called subscriber. The ringing relay isenergized only intermittently through the medium of the :interrupter240.- Upon the response of the called subscriber his line is providedwith talking current through the back-bridge relay 73, said circuitextending` from ground G12. through the springs 197, left-hand windingof the relay 73, over the heavy conductor 243, back contact of theringer relay springs, springs 242,

wiper 59, bank contact 2 4, over the heavy K trunk line, through thesubstation A, over the return trunk line, bank Contact 245, wiper 61,springs 246 and back contact ringer relay springs, conductor 247 and theright-hand winding of the relay 73 to battery B. The relayY 73, uponenergizing, closes the` circuit'of the ringer cut-oli' relay 248, which'circuit extends `from ground G21 through the springs 229 and 230,springs 249 ot the back-bridge relay 73, relay 248 and the side switchwiper 66 (in third position) to battery B. The relay 248, uponenergizing, interrupts the circuit of the ringer relay 7 4 at thesprings 241 and closes a locking circuit for itself through the springs250 independent of the springs 249. A further result. of theenergization of the back-bridge relay 73 upon the response of the calledsubscriber is the transposition of the battery connections o f thewindings of the line relay 72 with regard to battery B, whereby thedirection of the current in the calling line is reversed.

If a subscriber Whose line switch winding 12 is not shunted'by aresistance 203 should attempt to complete a connection with thesubscriber A., the marginal relay 62 would y the foiiowing circuit:Frein the busy signsitrunks i'roni which may lead to a selector oringninchine L through the sin-ings 252 and. e. connecter {not shown) inthe main 25lL oi'l the marginal relay 5,32, springs 21.2 end change.2li, side switch 'Der {Si} (in second pob'ince the operation of the lineswitch il 5 sition), condenser 'lover the heavy trunk and the selector Ehave hereinbeforf-z heen 70 conductor, ihr/oooh the ceiling substetiondcscrihed, i will non' assume thet- *he suhand nach over u ie heavyreturn conductor scrieer A has removed his receiver and roto the busysi.eenlingy machine 'L Should tilted his dial for the first digit 3,connect.- the ceiling subscriber'now attempt to cell ing the wipers. 25,26 and 27 of the first se- 1o the lest digit7 the shaft and shaft wiperslector E with the bank contact," 263, 2li-i and 75 would fail to rotate,as the rotary magnet 265. fis soon as the eonneetion is completedcircuit open at the springs 218 and 217 with the trunk conductors 266und 267, the o i the 62, the springs 2M line reioy 51toi the repeater lenergized. and 213 being open, es it Wili he remembered This circuitextends from ground G2, that these springs were opened by the shaftthrough the lower winding of the relay 5i, 50 upon reaching the tenthlevei. contact ot' springs 269 and 270 of the so- The release o t' theapparatus is' brought called hack-bridge relay 56, conductor 271, aboutby the hanging up of the receiver uttrunk conductor 267, 'bank contact265, Wiper the calling station in the following manner: 2?, over theheavy conductor, through the 0 lhen the receiver at substation A isIesubstation A, over the heavy return con 35 'stored to the switchhooh,the separation of ductor, wiper 25, bank contact 263, trunk thehook-switch springs l2() lill de conductor 266, conductor 273, springs27e stroys the energizingcireuit of the connector and 275 and the upperWinding, oi' the relay line relay 22.v 'rhe relay thereupon cle- 5e tobattery B. The relay 5%, upon enerl energised end opens oircuit ofthereiay gluing, closes a circuit extending freni 9o "95; yThe reiey N6,noon denergjlzing, ground G1" through the springs 276 and 277 t e holofthe reit/'S lll und the relay 57 to better. B. The relay 1 -J 57, uponenergizing, provides a holding rirc'eit for the relay O. Seid circuitextends from ground G28 through the righthand e5 Winding of the relay55, springs and 279, springs 280 and 281, over the ciniductor 272 si therelay C to the battery lend lOl). relay upon energizing, c npletes eholding; circuit for the relay 8J oi' tho seieci0() th and side switch'tor and the Winding l2 of the switch C. 1 s to .norniai position sind.also closes This circuit extends from ground G29 circuit extendingiron: nouncl i025 through through the springs 282 and 283, conductor t gct 2 -6 to the private bank Contact 268, bunk Contact 26% und the wiper2G to lo e of melting said contact the point 112. Here the circuitdivides, one 105 h' Yti e s 1 @turned to .normal branch extendingthrough the sln'ii s .Q1-li 3. "lne circuit of said relesseinagnet andand the relay 86 to tl e battery loud broken at the spr'ii js and 255when d10, the other branch extending through the the shaft reaches iLelowest position. The oiinorniel springs 93 and 92, springs 9i unddenergization of the reiey 171 of the selec- 20, hunk contact 21, Wiper17 and the wind- 110 tor F closes a circuit,through the release `ing i2to the battery lead 100. The relay D innonot This circuit extends fromulsoiproyides guarding ground potential ground (im through thesprings1.75 und extending ifroni ground G2 to the private 259, springs ...52,springs 261 and 261e, oi'- bank cont: et 264 and thence to the corre4 5Gnormal springs 262 and, release magnet spending contact in the bank ofeach selec- 115 258 to battery Theselector F releases, ter which hasaccess to the repeuter .l from thus breaking' its own release circuitwhen this suie-exchange.l The relay 57 also pro the shaft reaches itslowest position. The vides a guarding'potential extending from selectorE releases in'the seme manner as the ground G28 through the right-handwinding' kselector FI The i'vindingl 12 of the line of the relay 55,springs 278 and 279 and the 120 switch C, upon, inc coiningdenergized,elsprii'lgs 280 und 281 to the private bank conlonisthe plunger arm i"toreturn to normal tact 300, and thence to the correspondinn position.-contnct in the bank of euch selector which it will nowhi has access tothe repeater l from the eX- o if muy call as! "ine tcruiichange in whichSuid repeater is located. N5

notes' Therefore it will be seen that have pronedi 'ed a method formaking the truuk iii -1 1 c nuin exchange busj.Y to all culling suhinhotii sub-exchanges :is soon as the is seized. Another result of theenerlfm circuit. from ground nauseas 57 is the closure of a Grtt3through the rightthe coil 55, springsv 278 and 279, springs 280 and 281and the lefthand winding of the relay 56 to battery B. The relay 56 isso constructed and adjusted that it cannot be operatively energized byvone Winding alone, but vrequires the combined energization of both itswindings to cause it to attract its armature. rlhe engagement of thesprings 284 and 285, upon the energization of the repeater line relay54, closes a circuit through the line relay or a connector or selector(not shown) similar to the selector F or the' connector J located intliemain exchange, said circuit extending from ground through onewinding 'of the line relay, trunk conductor 286, springs 285 and284,1eft-hand winding of the impedance coil 55, right-hand winding ofthe relay 56, trunk conductor 287 and the opposite winding of the linerelay to battery. Although both windings of the'relay 56 of the repeaternowcarry current, the relay is not opera tiyely energized, because thecurrent flows .gization of the relay hand 'winding of through the twowindings in opposite direc! tions. rlhe apparatus is now in position forthe reception of impulses to complete the connection. The line relay 54of the repeater l operates in accordance with the impulses'o currenttransmitted from. the substation A and in turn repeats the impulses overthe conductors 286 and 287 to operate` the switches in the main oiice tocomplete the connection. These impulses are repeated through the mediumof springs 284 and 285 of the line relay 54 which open and close a shortcircuitacross the trunk conductors 286 and 287, the relays 55and 56being short circuited by the energization of slow acting re- -lay 58.The calling man releases this cone nection by hanging up his receiver,whereupon the line' relay 54 falls back and opens the circuit of theline relay of the switch ,y which it has selected, at the springs 284.and

285 allowing it to release in the usual manner. Relay 54 upondenergizing also opens the circuit of the slow acting relay 57, whichrelay in-turn falls back and opens the circuit of the back bridge relay56 and impedance coil 55 at the springs 280 and 281, andremoves' theguarding potential from the conductor 272. The circuit of the relay O isthereupon broken and this relay upon fall-.

' s ing back removes the holding ground from the selector E and lineswitch C, allowing them to release in the usual manner.

From the foregoing it will be seen that l have provided a telephoneexchange system iii/which a trunk line may extend through a number'ofexchanges and terminate in a ifnain exchange, and I have also providedmeans or'nialring said trunk line busy to exchanges when seized by asubscriber in any one or said exchanges. To accenti plish the aboveresult, relays O are placed in multiple with the third trunk conductorin the various exchanges. Said rela-ys are adapted to energize as soonas the trunk line is seized and operate to place a guarding potential onthe bank contact oitl every selector in the various exchanges that haveaccess to said trunk.

l have also provided and devised a very ei'iicient mechanism for makingreverting calls. rllhis is accomplished by allotting a special set ofnumbers to ea'cli subscriber on a party liiie'for use in ,calling othersubscribers on his line. rlhese numbers may be arranged on a card which`is placed at each subscribers phone. For example, if the subscriber. atA wishes tocall thesubscriber at A2 whose directory number is assumed tobe #2220 he will first consult the card of special numbers and iind whatnumber lie is to call .in order to connect with the proper combinationof generators which will ring the desired bells. He then operates hisdial in accordance with this number, and then, upon replacing hisreceiver upon the hook,

.his bell is sounded to advise him of the fact that the calledsubscriber is being signaled. rlhe cessation of this signal advises himof the fact that the called subscriber has answered. lt will also beseen that my mecha-` nisni controls circuits whereby the calledsubscribers bells may be rung a number ofi times, while the callingsubscribers bells need only to be rung once to notify him that thecalled subscriber is being signaled.

Furthermore, l have devised a connector switch which is in the nature ofa discriminating switch, z'. e., it allows certain subscribers to becalled by only certain other subscribers.

lt will also be seen that the release mag# nets of the selector E, theselective device H and the connector J are provided with means forproviding a guarding ground potential to their corresponding bankcontacts until said release magnets have denergized, which, occurs assoon as the switch shaft has returned to normal position.

While l have illustrated and described my invention in connection with aparticular type of automatic system in which wgllknown types or'automatic switches are einployed, itwill be readily understood thatvarious forms of switches can be employed without departing from thespirit of my invention and that various other changes and modificationscan be made. Likewise certain. features of my invention are adapted foruse in semi-automatic systems-tliat is, in systems where connection isextended in part. through the medium of a manual operators link circuit.

"` 1 at 'il claim as my invention irtv be Lrality ci? substationsthereon e v certain of said groups plurality of ringing generators atthe central ohce, one for eachsubstation, and means under the control ofa subscriber on said line for'connecting any two of said generators withthe line to alternately ring the signal at his own station and thesignal at one of the other. stations, said means including a ring J)ackapparatus having a single pair of line fwipei.

2. In a telephone system, a party-line, a ring-back apparatus associatedtherewith, said apparatus comprising one set of line wipers and aprivate wiper, a plurality 'of ringing generators, and means forconnecting a plurality of said generators alternately with said linethrough said line wipers.

3, In a telephone system, a party-line, a plurality of substationsthereon, corresponding series of ringing generators, one correspondingto each substation, a ring-back apparatus including a line wiper, meansfor connecting any pair of said generators with said line, and means forcausing ringing current from said selected generators to passalternately through said line wiper to said line.

1i. ln a telephone system, a trunk line, a pair of line terminals and athird conductor associated with each end of said trunk, means forseizing said line terminals, and means for vthen connecting one pole ofbattery in a iocal circuit with the third terminal at the distant end ofthe trunk, said means including a relay at the distant end and circuitfor said relay controlled from ,the other end over a circuit separatefrom the talking circuit.

5. ln a telephone system, a trunk line, a private conductor associatedtherewith, private contacts associated with each end of said trunk,means for seizing one end of said trunk, a relay at the distant end ofsaid trunk adapted to be energized over said trunk, means controlled bythe energization of said relay to ground said private conductor, andmeans controlled by the grounding of said conductor for connectingbattery locally with each of the third contacts at the opposite end ofsaid trunk. Y

6.l In a telephone System, a trunk line, means for extending aconnection to said trunk, a relay associated at the opposite end of saidtrunk adapted to be energized when said trunk is seized, and meanscontrolled by the energization of said relay for connecting batterylocally with the testing contact associated with each end of said trunk.

7. In a telephone system, a plurality of subscribers lines, an automaticconnector switch having a plurality of groupsiof line contacts, meansfor extending a connection from any 'of said subscribers lines to saidconnector, means for permittinE all of said subscribers to extendaconnectlon through contacts, and mea-ns assente for permitting only aportion of said suh scribers to extend a connection through other ofsaid groups .of contacts.

8. In a telephone system, a plurality of subscribers lines, an automaticconnector switch provided with a plurality of groups of bank contacts,causing the same to automatically select an idle contact in certain ofsaid groups, means for extending va connection from any of said lines tosaid connector, and means ior preventing certain of said subscribersfrom operating said. connector to select. an idle trunk in said certaingroups.

-9. ln a telephone system, plurality of subscribers lines, a line switchassociated with each of said lines, a relay associated with each of saidline switches, a resistance in shunt of said relay in certain of saidline switches, a connector switch, means for ex tending a connectionthrough any of said line switches to said connector switch, means forcausingone operation of said connector when connection is extendedthrough said line switches which are provided with such shunt, and meansfor preventing this operation of said connector when connection is made.with said connector through a line switch which is not provided withsuch shunt 10. In a telephone system, a plurality of subscriberslines,aline switch associated with each of said lines, a relay for each lineswitch, certain of said relays being provided with a shunt winding,a'connector, means for establishing connection with said connector fromany of said lines, means for causing said connector to select an idletrunk in a groupwhen it. has been raised to a cern tain level, and meansfor preventing such operation of said connector when connection isextended to the same through a line switch which isnot provided with ashunt.

11. In a telephone system, a group of subscribers lines, an individualprogressively movable trunking switch for each line, a scc ond group oflines, a connector for extending af'connection to said lines of thesecond grovp, said connector accessible in common to said individualswitches, means for rendering the connector inoperative after a line hasbeen trunked to the connector, and means for preventing the connectorfrom beingrendered inoperative after another ofsaid first lines hastiulnlred to the connector.

`l2. In a telephone system, a group of sub-V scribers lines, anindividual progressively movable trunking switch for each line, a secondgroup of lines, a lconnector for extending a connection to said lines ofthe second group, Vsaid connector accessible in common tosaidindividualswitches, means individual tothdconnecto'r for rendering theconnectorinoperative after a line has been trunked to the connector, andmeansindividual to the means in said connector for' 'ond group of lines,

connector-:ier preventing the connector from being rendered inoperativeafter another of said tirst lines has trunlred to the connector.

13. ln a telephone system, a group of subscribers lines, an individualprogressively movable trunlring switch for each line, a second group ci`lines, a connector for eX- tending a connection to said lines of thesecond group, said connectoraccessible in comnion to said individualswitches, means for rendering the connector inoperative after a line hasbeen trunked to Athe connector, and means for preventing the connectorfrom being rendered inoperative after another of said first lines hastrunlred to the connector, said last means in part individ .al to theconnector and in part to the said other line.

14C. in a, telephone system, a group of subscribers" lines, anindividual progressively movable trunlring switch for each line, asecond group ci lines, a connector for extending a connection to saidlines of the second group, said connector accessible in common to saidindividual switches, means for rendering the connector inoperative aftera line has been trunlted to the connector, and means for preventing theconnector from being rendered 'inoperative after another of said firstlines has trunked to .the connector, said last means comprising a relayindividual to the connector and a circuit for .said relay through aresistance in the individual switch allotted to the said other line. l5.ln a telephone system, a group of sub scribers lines, an individualprogressively movable trunking switch for each line, a seca connectori'oi extending a connection to said lines oi the second 'connector andgroup, said connector accessible in common to said individual switches,means for rendering the connector inoperative after a line Ihas beentrunlred tothe connector, means for preventing the connector from beingrendered inoperative after another of said first lines has trunked tothe connector', said last means comprising a relay individual to the acircuit 'or said rel'ay through a resistance in the individual switchallotted to the said other line, said resist-ance consisting of the,cut-bfi' relay of said indif vidual switch and a coil in multiplethereld in a telephone systema group of subscribersa lines, anindividual progressively movable trunlring switch for each. line, asecond group orq lines, a connector for extending 'a connection to saidlines of," the second group, an operating magnet circuit for saidconnector, said connector accessible in coininoii to said individualswitches, Vmeans for v rendering the connector inoperative after a-scribeis lines, an

lines has truiiked to the connector, Y

said last means comprising a relay individual y to the connector and acircuit for said relay through a resistance in the individual switchallotted to the said other line, said relay having under its controlsaid operating magnet circuit of the connector. i

17 ln a telephone system, a group ot subscribers lines, an individualprogressively. movable trunlring switch l'or each line, a second' groupof lines, a connector for extendin a connection to said lines ot thesecond group, said connector accessible in commonto said individualswitches means for rendering the connector inoperative after a line hasbeen trunked to the connector, and means for preventing said tirst meansfrom rendering the connector inoperative after another oif said firstlines has trunkedvto the connector.

18. In a telephone system, a group of subindividual progressivelymovable trunking switch for each lineq a second group ci' lines, aconnector for extending a connection to said lin-es 0i the second group,said connector accessible in common to said individual switches, i Aeansindividual to the connector for rendering the connector inoperativeafter a line has been trunked to the connector, and means individual tothe connector for preventing said first means from rendering theconnector. inoperative after another of said first lines has trunked tothe connector.

- 19. ln a telephones yst em, a group of submovable trunking switch 'foreach line, a second group'of lines, a connector for extending aconnection to said lines of the second group, said connector accessiblein common to said individual switches, means for rendering the connectorinoperative afterfa line has been trunked to the connector, and meanstor preventing said irst means from rendering the connector inoperativeafter another of said iii-st lines has trunked to the connec-l tor, saidlast means in part individual to the connector and in part to the saidother line.

20. ln a telephone system, a group of subscribers lines, an individualprogressively movable trunlring switch for each line, a sec ond group oflines, a connector for extending a connection to said lines of thesecond group. said connector accessible in common to said individualswitches, means for rendering the connector inoperative after a line hasbeen truhked to the connector, and means of said rst lines has trunlredto theconnec tor. said'last means comprising a relay individual. to theconnector and a circuit for said relay through a resistance in theindividual switch allotted to the said other line.

21. ln a telephone system, a group of sub-"5 scribeisvlines, anlindividual progressively nai-:asse flea 30. ln a telephod; system, aparty-line, a.

ring-back apparatu comprising a plurality of generators forenabling oneparty to call I another on the same line, andv meanscontrclled .by saidring-back apparatus for transmitting two kinds of ringing current backover said line to operate the calling' signal at both the calling and'called substations on said line, said means comprising a relayindividual to one of said generators, said ringback operated by currentover the line YiEor the purpose set forth. 4

31. ln a telephone system, a party-line, a ring-back apparatus forenabling one party to call another cn the same line, means forconnecting said line with said. apparatus, a plurality of ringingrgenerators for supplying ringing current of different characters, meanscontrolled by the calling subscriber for operating' said apparatus toselect any two of said generators, and means 'for then alternatelyt'ansmitting the current from said two selected generators over saidline, said means comprising a relay individual to each one of saidgenerators.

32. ln a telephone system, a party-line, a plurality of ringinglgenerators for supplying ringing; current of diierent characters, astep-by-step apparatus for selecting any two of said generators, andmeans for alternately supplying current from said selected generators tosaid line. said means comprising a relay individual to each one ci saidgenerators.

33. ln a telephone system, a party-line, a plurality of ringing`generators for supplying ringing current of different characters` astep-by-step ring-back apparatus, means for connecting said line withsaidapoaratus, means controlled by the calling subscriber foroperating,r said ring-back apparatus to select any two of said ringinglgenerators. and means for then alternately connecting said selectedringing generators with the line, said means comprising a relay indi- ,vyidual to each one of said geneators.

34. ln a telephone system', a party-line` a ring-baci; apparatuscomprising a plurality otygenerators for enabling one -party to callanothenon the same. line, and means ron- I trolled by said ring-bachapparatus for transmitting two kinds ot' ringing current back over saidline to operate the calling signal at both the callingand calledvsubstaticns on said line, said means comprising a relay individual toeachone ot' said tenerators. said ring back operated by current over theline for the purpose set `forth'.

35. ln a telephone system. a party-line, a ring-back apparatus forenabling one party to another on'the same line, a plurality of ringingcurrent generators, said ringback apparatus comprising a pair ofwipers,l

and means for connecting said wipers to thet'line and for thereafteralternately connecting said generators with said wipers.

36. In a telephone system, a party-line, a ring-back apparatus forenabling one party to call another on the same line, meansincludingr aselector switch for extending a connectlon from said line to saidring-back appara-tus, a plurality of sources of ringing current, meanscontrolled by said ring-back apparatus for "selecting any two ot' saidsources, and means controlled by the replacingf of the allingsubscribers receiver upon the switch-hook for alternately transmittingcurrent 4from said two selected generators over said line, said meanscomprising a relay individual to one of said generators. Y :jf-

37. ln a telephone system` a party line, a ring-back 'apparatus forenabling one party to call another onv the same line, means cluding aselector switch for extendingr a connection from said line to saidring-back .apparatus,a plurality of sources of ringing current, meanscontrolled by said ring-back apparatus for selecting any two of said.sources, and means controlled by the replacing of the callingsubscribers receiver upon the switch-hook for alternately trans-Knitting current from said two selected gen-` erators over saidline-saidmeans compris` ing' a relay individual to each one of said generators.

38. In a telephone system, a line, a second line, means for connectingsaid firstv and second lines. a plurality of signals operativelyassociated with vsaid first line, an automatic switch for said secondline` a signaling circuit or each signal under the control of saidswitch, means for supplying signaling current to said circuitscomprising a generator for each circuit and a relay individual to one ofsaid generators; and an impulse sender for operating said automaticswitch over said second line, to apply signaling current to said irstline, for operating' both said signals while the rst and second linesare connected, said relay controlling the application of signalingcurrent from said generator. .l

39. ln a telephone system, a line, a second line, means for connectingsaid first and second lines, a plurality of signals operativelyassociated with said first line, an autoing circuit for each signalunder the con- Ulti lit

trol of said'switch, means for supplying 4 signaling current to saidcircuits compris- 'ing a generator for each circuit and a relayindividual to each generator, andan 1mpulse sender. for operating vsaidautomatic switch over said second line to Vapply signalautomatic meanstor selecting ing current to said Erst line, for operating both saidsignals while the first and second lines are connected, said relayscontrolling tlie application of the signaling current to the line. v

40. ln a telephone system, a line, a second line, a pair of signals forsaid first line,

nccted line circuit while the two lines are connected. said relaycontrolling tlie'application of signaling current trom said generator.

ell., ln a telephone system, a line, a second line, a iair ol signals:for said first lino, means for connecting the lines, a- Signalingcircuit tor eacli signal, means for supply ing signaling current to saidsignaling circuits comprising generator for each circuit and :ii-relayindividual to each generator, and automatic means tor associating saidsignaling circuits with the connected line circuit wliile the two linesare connected, said relays controlling the application oli the signalingcurrent to the' line,

ll-2. ln a telephone system, a line, a second line, a plurality ofsignals for said first line, means lt'or connecting the two lines, asignaling circuit for each signal, means for supplying signaling currentto said circuits comprising generator for each circuit and relayindividual to one of said. generators, and automatic means for selectingany two ol said circuits and for associating the selected signalingcircuits with the connec-ted. line circuit while the said two linecircuits are connected, said relay controln ling the application ofsignaling current 'from generatorn ln a telephone system, a line, asecond line, plurality of'signals for said irst line, `means forconnecting tlie two lines, a circuit tor eacli signal, means forsupplying signaling current to said circuits comprising a generator foreach circuit and a. relay individual to eaeli generator, and ny two ofsaid circuits and 'for associatin lie selected signaling circuits withthe ciected line circuit while the said two line ,circuits areconnected, said relays controlling tlie applil cation of the signalingcurrent to the line.

fla. ln a telephone system, a line, a second line, a plurality ofsignalsfor saidV iirst line, means for connecting the itwo lines, a signalingcircuit for each signal, meansr for supp ying signaling current to saidcircuits comprising a generator for each circuit and relay individual toone of'said generators, und aigtcmaticameans for selecting a pluissirrality of said generators and for assoc1ating the selected generatorswith the said first line while the said'lines are connected, said relaycontrolling the application of signal ing current from one of saidgenerators to the said first lin-e.

l-. ln a telephone system, a line, a second line, a plurality of signalsfor said first line, means for connecting tlie two lines, a signalingcircuit for each signal, means for supplying signaling current 'to saidcircuits comprising a generator for each circuit and a relay individualto each generator, and automatic means l'or selecting any two of saidcircuits and for associating the selected signaling circuits with theconnected line circuit while the said two line circuits are connected,said relays controlling the application of 'the signaling currentv tothe line.

a6. ln a telephone system, a line, a second line, automatic means forconnecting said iirst and second lines, a plurality of signalsoperatively associated with said first line, an automatic switch forsaid second line, a signaling circuit for each signal under tlie controlof said switch, means for supplying signaling current to said circuitscomprising a generatorfor eacli circuit and a relay individual to one ofsaid generators, an impulse sender for operating said automaticswitchover said second line, to apply signaling current to said irst line, foroperating both said'signals while the first and second lines` areconnected, said relay controlling the application of signaling currentfrom said generator, said impulse send-er also for controlling saidautomatic means over tie second circuit for the purpose set forth. n

4.7. ln a telephone system, a line, a second line, automatic means forconnecting said first and second lines, a plurality of signalsoperatively associated witli said first line,

an automatic switcli for said second line, a

signaling circuit for each signal under the control ot said switch,means for supplying signaling current to said circuits comprising agenerator tor eacli circuit and a relay individual to each generator,and an impulse sender for operating said automatic switch lll) lili

over said second line to a i ily sienalinrr cury l? D ront to said firstline, for operatingboth said signals gwliile the first and second linesare connected, said relays controlling the application ci the signalingcurrent to the line, said impulse sender also for controlling saidautomatic means over the second circuit tor the purpose set forth,

48. In a telephone system, a line, a second line, a. pair of signals forsaid first line, means for connecting ille lines, a signaling circuitfor eacli signa-l, means for supplying signaling current to saidsignaling circuits comprising a generator for each-circuit and a relayindividual to one of said generators, and

lill) automatic means for associating 'said signaling circuits with theconnected circuit while the two lines are connected, said relaycontrolling the application ot' signaling current from said generator,and an impulse sender for controlling said automatic means over thesecond circuit for the purpose set forth.

49. ln a telephone system, a line, a second line, a pair of signals .forsaid first line, means for connectingthe lines, a signaling circuit toreach signal, means for supplying signaling current to said signalingcircuits comprising a generator for each circuit and a relay individualto each generator, and automatic means for associating said signalingcircuits with the connected circuit while the two lines are connected,said relays controlling the application of the signaling current to theline, and an impulse sender Jfor controlling said automatic means overthe second circuit for the purpose set forth.

50. ln a telephone system, a line, a Second line, a plurality of signalsfor said first line, means for connecting the two lines. asignalingcircuit for each signal, means for supplying signaling current to saidcircuits comprising a generator for each circuit and a relay individualto one of said generators, and automatic means for selecting any two ofsaid circuits and for associating the selected signaling circuits aviththe connected circuit while the said two line circuits are connected,said relay controlling the application otl signaling current from saidgener ator, and an impulse sender for controlling said automatic meansever the second circuit tor the purpose set forth.

51. ln a telephone system, a line, a second line, a plurality of signalsfor said first line, means tor connecting the two lines, a signal- `ingcircuit for each signal, means for supplying signaling current to saidcircuits comprising a generator for each circuit and a relay individualto one of said generators, and automatic means for selecting a pluralityot said generators and for associating the selected generators with thesaid first linev while the said lines are connected, said relaycontrolling the application of signaling curreiit from one of saidgenerators to the said tirst line, said automatic means operated overthe connected lines.

ln a. telephone system, a line, a second line, a plurality of signalsfor said first line, means for connecting the two lines, a signalingcircuit for each signal, nieans'for supplying signaling current to saidcircuits comprising a generator for each circuit ,and a relay individualto one ot said generators, and automatic means for selecting apluralityot said generators simultaneously and for associating theselected generators with the said first line while the said lilies areconnected, said relay controlling the appliation ot signalingcurrentfrcm one caaid generators to the said first line, said automatic'mea-ns operated over the connected lines.

"53. in a telephone system, a plurality otsubscribers lines, a lineswitch associated with each of said lines, a relay associated with eachof said line switches, a resistance in shunt oi said relay in certain otsaid line switches, a connector switch, means for eX tending aconnection through any of said line switches to said connector switch,electromagnetic means for causing one operation of said connector whenconnection is extended through said line switches which are providedwith such shunt, and a connector controlled switch for preventing thisoperation of said connector when connection is made .with said connectorthrough a line switch which is not provided with such shunt.

541-. In a telephone system, a plurality or' subscribers lines, a lineswitch associated witlieacli of said lines, a relay `for each lineswitch, certain of said relays being provided with a shunt winding, aconnector, means for establishing connection with said connector fromany of said lines, means for causing said connector to select an idletrunk in a group when it has been raised to a certain level, and aconnector controlled switch for preventing such operation of saidconnector when connectionis extended to the same through a line switchwhich is not provided with a shunt.

;.111 a telephone system, a group of subscribers lines, an -individualprogressively movable trunking switch -for each line, a second group oflines, extending a connection to said lines ot' the second group, saidconnector accessible in common to said individual switches, a connectorcontrolled switch for rendering the connector inoperative after a linehas been trunlzed to'the connector, and electromagnetic means forpreventing the connector` from being rendered inoperative after anothero'f said iirstlines has trunked to the connector.' I y 56. In atelephone system,'a group of suhscribersl lines, an individualprogressively movable trunking switch for eachA line, a second group ot'lines, a connector tor extending a connection to said lines of thesecondgroup, said connector accessible in common to said individual switches,a connectorV controlled switch individual to the connector for renderingthe connector inoperative after a line has been trunked tothe connector, and electromagnetic means individual to-tlie connector forpreventing the connector from being renderedv inoperative after anotherofsaid first lines has trunlied to the connector.

57. ln a telephone systenn'a group of sub.- scriloers lines, anindividual progressively a connector for

